(1) Field of the Invention
The present invention generally relates to a fuel injection control apparatus, and more particularly to an apparatus for controlling a fuel injection time for a fuel injector in response to operating conditions of an internal combustion engine.
(2) Description of the Related Art
If an internal combustion engine continuously operates in a high load region, exhaust parts (e.g., a catalytic converter) are heated by exhaust gas from the engine and the temperatures of the exhaust parts increase to a high level. When the exhasut gas temperature exceeds a certain high temperature and the engine is still operating within the high load region, the exhaust parts may be damaged due to the heat of exhaust gas. Generally, in order to avoid damaging the parts, a fuel injection time, during which fuel is injected to the engine by a fuel injector, is increased so that the fuel injection sends a more rich air-fuel mixture to the engine when it is detected that the operating conditions of the engine lie in a prescribed high load region. The increase of the fuel injection time is called hereinafter the over-temperature protect (OTP) process, and when a fuel injection control apparatus performs this OTP process the fuel injection amount is increased. If the fuel injection amount is increased so as to send a rich air-fuel mixture to the engine immediately when the engine is detected as operating in the high load region, it is possible to prevent an undesired increase of the exhaust gas temperature to a cetain extent, due to cooling after the heat of fuel vaporization is consumed, and due to a decrease of combustion efficiency accompanied by a decrease in amount of oxygen gas in the air-fuel mixture. However, because the exhaust parts have a certain heat capacity, the exhaust parts are not immediately damaged by the heat of exhaust gas when the engine is operating in a prescribed high load region. The above mentioned OTP process is usually performed after a given delay time has elapsed during which the engine operates in the prescribed region, so that the fuel injection amount is increased after the exhaust part temperatures have increased to almost the same level as the exhaust gas temperature.
Generally speaking, a difference between the exhaust part temperature and the part damage temperature becomes smaller when a higher exhaust part temperature is detected when the engine is operating in a prescribed high load region. Thus, if the detected exhaust gas temperature is high, the time for the exhaust part temperature to reach the part damage level at which the exhaust parts are damaged due to the exhaust gas heat becomes short.
In the prior art, there is an apparatus for controlling a fuel injection time in response to operating conditions of an engine so that the overheating of exhaust parts, after an engine load (detected from the engine operating conditions) higher than a prescribed level is detected, is prevented. For example, Japanese Laid-Open Patent Publication No.60-43144 discloses such an apparatus. In this conventional apparatus, an exhaust gas sensor is mounted in an exhaust pipe of an engine. When it is detected that the engine load is higher than a prescribed level, an exhaust gas temperature is sensed by the exhaust gas sensor. In this system, the delay time between detection of the high engine load and increasing of the fuel injection time is varied in response to the sensed exhaust gas temperature. More specifically, the delay time is decreased when the exhaust gas temperature is high, and when the exhaust gas temperature is low the delay time is increased.
However, the exhaust gas temperature measured by the sensor when the engine load is higher than a prescribed level does not represent correct temperatures of exhaust parts at that time. Becuase the exhaust parts have a certain heat capacity, the temperatures of the exhaust parts increase to the measured exhuast gas temperature slightly after the exhaust gas temperature just measured by the sensor. In other words, there is a delay between detection of the exhaust gas temperature and increase of the exhaust part temperatures. If the exhaust gas temperature measured when the engine load is higher than a prescribed level is at the same level, the exhaust part temperatures at that time are different depending on the heat energy having been given to the exhaust parts prior to the detection of the high engine load condition. An exhaust part temperature is obviously higher when a great amount of heat energy has been given than when a small amount of heat energy has been given. Thus, if the above described delay time is changed to a smaller value when the exhaust gas temperature is found to be high, as in the above mentioned conventional apparatus, there is a problem in that the increase of the fuel injection time is performed immediately although the exhaust part temperature has not yet increased to the measured exhaust gas temperature, and it is not yet necessary at that time to change the fuel injection time. Therefore, due to an undesired increase of the fuel injection amount, the fuel efficiency is decreased.